Critical angle and total internal reflection
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- A mirror made of glass
- The set-up
- Three cases
- The definition
Read the transcript of the teaching part
Last time
Before we start: last time, in Waves, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A mirror made of glass
A glass prism can act like a perfect mirror. No silver coating, yet every bit of the light bounces back. This is total internal reflection, and it carries phone calls and internet data around the world through thin strands of glass.
The set-up
Light travelling inside glass towards the air boundary is going from a slower medium to a faster one. It bends away from the normal. As the angle of incidence gets bigger, the angle of refraction gets bigger faster. At some angle the refracted ray reaches 90 degrees, along the surface.
Three cases
Below a certain angle of incidence, called the critical angle, most of the light leaves the glass and a little is reflected. At exactly the critical angle, the refracted ray travels along the surface. Above it, no light escapes at all. All of it is reflected back inside, with the angle of reflection equal to the angle of incidence.
The definition
The critical angle is the angle of incidence in the denser medium at which the angle of refraction is 90 degrees. Total internal reflection needs two conditions: the light must be going from a denser to a less dense medium, and the angle of incidence must be greater than c. Both conditions. Missing either, and there is no total internal reflection.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Total internal reflection · Critical angle and total internal reflection.
Chapter 3.2: Light
- Reflection by a plane mirror 7 min
- Refraction and refractive index 5 min
- Critical angle and total internal reflection 5 min
- Thin lenses, ray diagrams and magnification 6 min
- Uses of lenses and correcting sight 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.